A typical photoresist lithography process used in the fabrication of a semiconductor operates as follows. A layer of material to be patterned is formed on a substrate. The layer to be patterned may be polysilicon, polycide, silicon oxide, nitride or metal. Then, a layer of photoresist is deposited on the layer to be patterned. Thereafter, the photoresist is exposed in selected regions with light through a mask. The photoresist is then removed in the regions other than the selected regions. Then, a process can be performed in the underlying layer through the removed portions of the photoresist, such as removing portions of the underlying layer to be patterned.
The thickness of the photoresist layer varies considerably as a function of its position on the substrate surface, especially during the back-end process steps in the fabrication of a silicon wafer. Referring to FIG. 1, such differences in photoresist thickness are represented by t.sub.1 and t.sub.2. In this cross-section view, the photoresist (PR) layer covers a polysilicon or polycide (poly) layer in preparation for the lithographic process. When light (illustratively ultra-violet) is directed at the photoresist surface, approximately 50% of the incident beam is reflected back into the photoresist from the poly layer. As indicated in FIG. 2, incident light uv.sub.i and reflected light uv.sub.r combine in the PR layer to create interferences, or standing waves.
Thus, when the photoresist is exposed to incident light during the lithographic process, standing waves occur in the photoresist, which cause distortions in the patterns on the underlying layer, as determined by the thickness variations in the photoresist. For example, as illustrated in FIG. 3, ultra-violet light 10 is directed through a mask 20 to the photoresist layer, which is polymerized in the areas (30,40) not covered by the mask. Ideally, these polymerized areas are uniformly aligned with their respective mask openings, as indicated by the width w in FIG. 3, which assumes minimal impact from standing waves in the t.sub.1 thickness region of the PR layer. In the t.sub.2 thickness region, however, the standing wave effect causes a distortion in this polymerized area, as indicated by width w.sub.pr. Since the function of the polymerized PR is to form a mask for patterning the underlying poly layer, any distortions in the PR mask are replicated in the poly layer patterns. For example, necking and bridging are two types of distortions caused by the standing wave effect, as depicted in the top view of a poly layer in FIG. 4. Such pattern distortions are detrimental to both yield and performance of semiconductor devices, and become an increasingly serious problem as the scale of VLSI devices diminishes.
The above described standing wave effect of the photoresist results in necking and bridging of adjacent pattern lines in the underlying poly (or metal) layer. The standing wave effect becomes a serious production problem as the scale of VLSI fabrication continues to decrease.
In the prior art, an additional layer of anti-reflecting material has been used between the photoresist and the underlying layer. For metal layers, where the reflectivity is near 100% for ultra-violet light, the standing wave problem has been solved by depositing a thin layer of anti-reflecting material on the metal surface. This reduces the reflectance of the metal layer to approximately one fifth of its uncoated value.
For polysilicon or polycide layers, where the reflectivity is about 50% for ultra-violet light, a thin organic resist-like layer has been used in various ways, including top-coating and bottom coating, after and before layering the photoresist. These techniques have numerous disadvantages, however, such as process complexity, layer stripping, and contamination.
It is an object of this invention to overcome the aforementioned disadvantages when using polysilicon or polycide layers in the VLSI fabrication process. It is a further object of this invention to improve the product yield and performance of VLSI devices by using anti-reflecting materials of proven reliability.